1. Technical Field
The invention relates generally to systems for measuring birefringence or other optical property, e.g., transmission, of a sample of material.
2. Background Art
Birefringence, or double refraction, is a phenomenon that occurs in materials characterized by two indices of refraction. Typically, birefringent materials are optically anisotropic substances, e.g., calcite and quartz, although some isotropic materials such as glass and plastic become birefringent when subjected to stress. When a beam of light enters a birefringent material, the beam splits into two polarized rays each traveling at a different velocity, corresponding to a different index of refraction. One ray, called an ordinary ray, is characterized by an index of refraction that is the same in all directions. The second ray, called an extraordinary ray, travels with different speeds in different directions and hence is characterized by an index of refraction that varies with the direction of propagation. If the light entering the birefringent material is unpolarized or linearly polarized, the ordinary and extraordinary rays will have the same velocity along one direction, called the optic axis. The ordinary and extraordinary rays recombine upon exiting the material.
Birefringent materials can change the polarization state of a light passing through them. Therefore, the ability to accurately determine the birefringence of a sample is important, especial y in high performance optics, e.g., ophthalmic lenses, laser optics, and optical fiber, where a change in the polarization state of light can cause dramatic changes in optical performance. When linearly polarized light passes through a birefringent sample, the sample rotates the direction of polarization through some angle. By measuring this angle of rotation, the birefringence of the sample, i.e., the difference between the highest and lowest indices of refraction of the sample, can be determined. Typically, the sample is placed between two crossed linear polarizers. The birefringence at a given point about the cross section of the sample is then determined by measuring the angular position, with respect to the first linear polarizer, at which the light emerging from the sample is extinguished as it passes through the second linear polarizer.
Various other methods are known for determining birefringence. One example of a known method is disclosed in U.S. Pat. No. 5,257,092 issued to Noguchi el al. As shown in FIG. 1, an optical source unit 2 emits a linearly polarized light beam, which passes through a quarter-wave plate 4. The quarter-wave plate 4 converts the beam emitted by the optical source 2 to circularly polarized light, which then passes through the birefringent sample 6, where the light emerges elliptically polarized. This emergent light then passes through a second quarter-wave plate 8 which converts the light to near-linear polarized light. The light then passes through a rotatable analyzer 10. Birefringence is determined by measuring the angle of the analyzer 10 with respect to the source 2 at which light is extinguished. The method disclosed by the Noguchi et ail. ""092 patent uses circularly polarized light rather than linearly polarized light because, in the samples used, birefringence had to be measured in all directions. If linearly polarized light is used, there inherently will be a direction in which no birefringence occurs, i.e., the optic axis.
Another example of a method for measuring birefringence is disclosed in U.S. Pat. No. 5,587,793 issued to Nakai el al. As illustrated in FIG. 2, a sample 12 is placed between a circular polarizer 14 and a circular analyzer 16 and arranged in an optical path between a light source 18 and an optical receiver 20. The circular polarizer 14 is a combination of a polarizer 22 and a quarter-wave plate 24, and the circular analyzer 16 is a combination of a quarter-wave plate 26 and an analyzer 28. The circular analyzer 16 is arranged in a crossed Nicols fashion with respect to the circular polarizer 14. A crossed Nicols fashion refers to the arrangement of the polarizers such that their polarization axes are set 90 degrees from one another. In this method, monochromatic parallel beams emitted from the light source 18 are converted into circularly polarized light by the circular polarizer 22 and projected onto sample 12. The light beams then pass through the circular analyzer 16 to be detected by the optical receiver 20.
The birefringence of the sample may vary from location to location across the sample. Thus, in order to describe the birefringence of a sample, birefringence at a number of points along or distributed on the surface of the sample is measured. One procedure used in industry includes taking a measurement at one position on the cross section of a sample and then manually moving the sample e.g., by using a lab jack, so that the measurement is made at another test point on the cross section. The measurements are repeated at numerous test points about the cross section of the sample to generate a birefringence map. Because mapping requires a large number of points, mapping the sample manually is a difficult and time-consuming task. In some cases, the actual birefringence measurement is also performed manually, with the operator having to determine the actual angle of light extinction. Therefore, the accuracy of these measurements can vary from operator to operator.
The invention is a method for automating measurement of an optical property of a sample. The method comprises selecting a measurement aperture around a reference point on the sample, generating a set of grid nodes that fall within measurement aperture, calculating the radial distance of each node with respect to a reference point within the measurement aperture, and calculating the angular position of each node with respect to the vertical. The method further includes calculating the angular position of each node with respect to the vertical and moving a light source and a light detector along the vertical and rotating the sample to measurement positions within the measurement aperture. The calculated radial distances and angular positions are used to control positioning of the light source and the light detector and rotation of the sample. The optical property is measured at the measurement position by energizing the light source and interrogating the detector.